Technical Field
The present disclosure relates to the field of applied geophysics, and in particular, to a mine transient electromagnetic (TEM) three-component detection method applied to advanced detection for electrically anisotropic geology in a mine roadway.
Description of the Related Art
A TEM technique has been widely used in prevention and control of water inrush in coal mines, which reduces the occurrence of water inrush accidents in the coal mines, reduces economic losses, and ensures the safety of people. The TEM technique has a broad development prospect, which is sensitive to low-resistance anomalies, and can find out water-containing geological bodies such as karst caves, tunnels, mined-out areas, deep irregular aquifers, and the like. The TEM technique can implement deep exploration; and has advantages such as sensitivity to low-resistance geological bodies, good coupling with a detected target, strong response to anomalies, simple form, strong discrimination, and the like. As coal mining proceeds deeply and longitudinally, geological conditions become relatively complicated, and the electrical anisotropy of the geology in the mine is serious. In this case, the TEM detection technique faces great challenge, because the TEM detection theory based on horizontally layered media determines the spatial location of the anomaly in front by detecting secondary field information of only one component.
The TEM method is also called time-domain electromagnetic method, which is a technique based on the theory for horizontally layered media. In this technique, a primary pulsed magnetic field is emitted into the earth using an ungrounded loop or ground source, and during an interval of emission of the magnetic field, a secondary turbulence field produced by the anomaly is observed using a coil or ground electrode. The change pattern of the secondary field as a function of time in multiple time periods after interruption of power supply is investigated, that is, the induced voltage is measured. Based on the electromagnetic field theory, the measured data is converted into information about formation depth and resistivity, to obtain geo-electrical features of the formation at different depth.
Due to the serious electrical anisotropy of the geology in the mine, the TEM detection technique based on the theory for horizontally layered media faces great challenge in advanced detection in a roadway.
Currently, advanced detection in mine roadways conducted according to the TEM is based on the theory for layered formation, and only the normal component of the heading face is measured. For electrically anisotropic formation, geological anomalies are determined only according to data about the normal component, resulting in a seriously wrong geological conclusion. For example, a fault fracture zone of a steep vertical fault is a good groundwater storage zone, and its model can be analogized according to the geo-electrical model FIG. 11. During the advanced detection for the vertical fault fracture zone, a primary field is hardly cut, and thus an induced secondary field is rather weak. It is rather difficult to infer, according to the weak secondary field, whether there is a low-resistance fault fracture zone ahead of the heading face.